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But in the last two paragraphs of his statement he referred to a historic agreement dating back almost a century which acknowledged Chinese interest in Tibet but asserted that Tibet had never been fully part of the country.

He described it as an "anachronism" and added: "Like every other EU member state, and the United States, we regard Tibet as part of the People's Republic of China."

The change in position is being attacked by a growing coalition of academics, Tibet support groups and the Tibet government-in-exile itself.

Thubten Samphel, the government-in-exile's spokesman, said it was "greatly disappointed". "For the British Government to change its position at this stage to us seems counter-productive," he said.

Britain's position derives from its colonial history – a reason why ministers and the Tibetan movement itself have rarely emphasised it.

The Simla accords of 1913 set the boundary between Tibet and British-ruled India.

They reflected the fact that Tibet had fallen within first the Mongolian and then the Chinese military orbit in previous centuries but had mostly governed itself. Britain was said to recognise Chinese "suzerainty" but not "sovereignty" over the region.

While the distinction might be obscure, it meant there was a basis in international law, backed by a permanent UN Security Council member, for Tibet to be recognised as distinct from other "provinces" of China.

Mr Miliband said this distinction, and the whole idea of "suzerainty" was outdated.

"Some have used this to cast doubt on the aims we are pursuing and to claim that we are denying Chinese sovereignty over a large part of its own territory," he said.

He was supported by Lord Patten, the last British governor of Hong Kong. He told the Foreign Correspondents Club of China at the weekend that the position was a "quaint eccentricity".

But the Free Tibet Campaign and the International Campaign for Tibet fear the change has cut the ground from under the Dalai Lama's feet.

The ICT called the sudden change "baffling and unfortunate". The Free Tibet Campaign said the Government was "rewriting history".

The timing could not be more sensitive. Many of the issues being discussed between Beijing and the Dalai Lama's representatives, such as the boundaries of Tibet and the extent to which it is allowed to handle its own affairs, are exactly the same as those addressed by the Simla accords.

Most strikingly, Britain's position in the accords, repeated since, was that its recognition of Chinese "suzerainty" was dependent on China granting Tibet political autonomy.

Robbie Barnett, a British historian of Tibet at Columbia University in New York, said that Mr Miliband's statement stressed Britain's concern for human rights in Tibet but gave away the only leverage the outside world had to influence events there.

"This is more than a bargaining chip," he said. "This is the entire legal and political foundation for these talks."

The Foreign Office insists that there has been no change in policy, and that Mr Miliband was merely "clarifying" its current position.

A spokesman refused to be drawn on whether Britain had been offered or asked for anything in return for its concession to Beijing.

She confirmed that the Chinese were “glad” when informed by the British Ambassador to China, Sir William Ehrman, but added: “We did not give in to Chinese pressure. China was not pushing us on this.”

Stephanie Brigden, director of the Free Tibet Campaign, said Britain had given away a bargaining chip in return for absolutely nothing.

”It’s extraordinary that Britain has rewarded China in such a way in the very year that China has committed some of the worst human rights abuses in Tibet in decades, including torture and killings,” she said.